Method for manufacturing a metal sheet with a znal coating and with optimised drying, corresponding metal sheet, part and vehicle

ABSTRACT

The present invention provides a method for manufacturing a metal sheet. In this method, at least one of the following equations is satisfied: 
     
       
         
           
             
               
                 
                   
                     
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             wherein: 
             Z is the distance between the metal sheet and the nozzle along the main ejection direction (E), 
             Z being expressed in mm, 
             d is the average height of the outlet of the nozzle along with the running direction of the metal sheet in front of the nozzle, d being expressed in mm, 
             V is the running speed of the metal sheet in front of the nozzle, V being expressed in m·s −1 , 
             P is the pressure of the wiping gas in the nozzle, P being expressed in N·m −2 , and 
             fO 2  is the volume fraction of oxygen in the wiping gas. A metal sheet, part and land motor vehicle are also provided.

BACKGROUND

The present invention relates to a method for making a metal sheetcomprising a steel substrate, at least one face of which is coated witha metal coating comprising Al, the remainder of the metal coating beingZn and inevitable impurities.

Such a metal sheet is more particularly intended for making body partsfor a land motor driven vehicle such as an automobile.

The metal sheet is then cut out and deformed in order to form the bodyparts or the body.

This body is then coated with a film of paint (or paint system) whichensures a good aspect of the surface and participates with the metalcoating based on zinc, in protection against corrosion.

Coatings based on zinc of metal sheets have what is called a waviness oftheir outer surfaces, which can presently only be compensated bysignificant thicknesses of paint, under the penalty of having aso-called “orange peel” aspect, unacceptable for body parts.

The waviness W of the outer surface of a coating is a smoothpseudo-periodic geometrical irregularity with quite long wavelength (0.8to 10 mm) which is distinguished from roughness R which corresponds togeometrical irregularities with short wavelengths.

SUMMARY OF THE INVENTION

In the present invention, the arithmetic mean Wa of the wavinessprofile, expressed in μm, was retained for characterizing the wavinessof the outer surface of a metal sheet coating, and the waviness ismeasured with 0.8 mm a cut-off threshold and designated by Wa_(0.8).

A reduction in the waviness Wa_(0.8) may allow reduction of thethickness of the paint film used for attaining a given property of paintaspect or, for constant thickness of the paint film, an improvement inthe quality of the paint aspect.

The present invention provides a method for making a metal sheet,comprising a substrate, for which at least one face was coated by dipcoating with a metal coating based on zinc and comprising between 0.2and 0.7% by weight of Al, the outer surface of the metal coating havingreduced waviness Wa_(0.8).

For this purpose, the present invention provides a method formanufacturing a metal sheet comprising a steel substrate at least oneface of which is coated with a metal coating comprising Al, theremainder of the metal coating being Zn, inevitable impurities andoptionally one or more additional elements selected from Si, Sb, Pb, Ti,Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the weight content of each additionalelement in the metal coating being less than 0.3%, the metal coatinghaving a weight content of aluminum comprise between 0.2 and 0.7%. Themethod comprising the steps of providing the substrate, depositing ametal coating on at least one face by dipping the substrate in a bathfor obtaining the metal sheet, wiping the metal coating with at leastone nozzle projecting through at least one outlet a wiping gas on themetal coating, the metal sheet running in front of the nozzle, thewiping gas being ejected from the nozzle along a main ejection directionE and solidifying the metal coating. The outer surface of the metalcoating has, after solidification and before an optional skin-passoperation, a waviness Wa_(0.8) of less than or equal to 0.55 μm. Atleast one of the following equations is satisfied in the method:

$\begin{matrix}{{\frac{Z}{d} + {18\; {\ln \left( \frac{Z}{d} \right)}}} < {{8\; {\ln \left( \frac{P}{V} \right)}} - 27.52}} & (A) \\{{fO}_{2} < \frac{2.304 \cdot 10^{- 3}}{\left( {27.52 + \frac{Z}{d} + {8\; {\ln \left( {\frac{V}{P}\left( \frac{Z}{d} \right)^{2.25}} \right)}}} \right)^{2}}} & (B)\end{matrix}$

wherein:Z is the distance between the metal sheet 1 and the nozzle along themain ejection direction E,Z being expressed in mm,d is the average height of the outlet of the nozzle along the runningdirection S of the metal sheet in front of the nozzle, d being expressedin mm,V is the running speed of the metal sheet in front of the nozzle, Vbeing expressed in m·s⁻¹,P is the pressure of the wiping gas in the nozzle, P being expressed inN·m⁻², andfO₂ is the volume fraction of oxygen in the wiping gas.

The method may also comprise the following features, taken individuallyor as a combination:

the waviness Wa_(0.8) of the outer surface of the metal coating (7)after solidification and before an optional skin-pass operation is lessthan or equal to 0.35 μm;

before the deposition step, a step for cold rolling the metal sheet, atleast the last pass being achieved with rectified and non-etched workrolls for which the work surfaces have a roughness Ra_(2.5) of less thanor equal to 0.5 μm;

a step for a skin-pass of the metal sheet with EDT work rolls, theworking surfaces of which have a roughness Ra_(2.5) comprises between2.05 and 2.95 μm;

a step for skin-pass of the metal sheet with EBT work rolls, the workingsurfaces of which have a roughness Ra_(2.5) comprised between 2.90 and4.10 μm;

satisfying at least one of the following equations:

$\begin{matrix}{{\frac{Z}{d} + {18\; {\ln \left( \frac{Z}{d} \right)}}} < {{8\; {\ln \left( \frac{P}{V} \right)}} - 36.32}} & (C) \\{{{fO}_{2} < \frac{2.304 \cdot 10^{- 3}}{\left( {36.32 + \frac{Z}{d} + {8\; {\ln \left( {\frac{V}{P}\left( \frac{Z}{d} \right)^{2.25}} \right)}}} \right)^{2}}};} & (D)\end{matrix}$

the metal coating has a weight content of aluminum less than or equal to0.6%; and

the metal coating has a weight content of aluminum less than or equal to0.5%.

The present invention also provides a metal sheet comprising a steelsubstrate, at least one face of which is coated with a metal coatingcomprising Al, the remainder of the metal coating being Zn, inevitableimpurities and optionally one or more additional elements selected fromamong Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the weightcontent of each additional element in the metal coating being less than0.3%, the metal coating having a weight content of aluminum comprisebetween 0.2 and 0.7%, the outer surface of the metal coating having awaviness Wa_(0.8), before an optional skin-pass operation, of less thanor equal to 0.35 μm.

The metal sheet may have a metal coating that has a weight content ofaluminum less than or equal to 0.6% and preferably less than or equal to0.5%.

The present invention further provides a part obtained by deformation ofa metal sheet according to the present invention, wherein an outersurface of the metal coating has a waviness Wa_(0.8) of less than orequal to 0.43 μm.

The part may include a waviness of the outer surface of the metalcoating that is Wa_(0.8) less than or equal to 0.41 μm or preferablyless than or equal to 0.37 μm.

The part may include a film of paint on the metal coating. The film ofpaint may be less than or equal to 120 μm, or preferably less than orequal to 100 μm.

The present invention provides a land motor vehicle comprising a body,the body comprising a part according to present invention.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be illustrated by examples given as an indication,and not as a limitation, and with reference to the appended figureswherein:

FIG. 1 is a schematic sectional view illustrating the structure of ametal sheet according to the present invention,

FIG. 2 is a schematic side view illustrating a tank and wiping nozzlesfor making the metal sheet of FIG. 1,

FIG. 3 is a partial, schematic and enlarged view of the circled portionIII of FIG. 2, and

FIG. 4 is a schematic view taken along the arrow IV of FIG. 3, andillustrating the shape of the output of the nozzle of FIG. 3.

DETAILED DESCRIPTION

The metal sheet 1 of FIG. 1 comprises a steel substrate 3 coated on eachof its two faces 5 with a metal coating 7.

It will be observed that the relative thicknesses of the substrate 3 andof the different layers of coating have not been observed in FIG. 1 inorder to facilitate the illustration.

The coatings 7 present on both faces 5 are similar and only one will bedescribed in detail subsequently. Alternatively, only one of the faces 5has a coating 7.

The coating 7 generally has a thickness of less than or equal to 25 μmand aims at protecting the substrate 3 against corrosion.

The coating 7 comprises zinc and aluminum. The aluminum weight contentof the metal coating 7 is comprised between 0.2 and 0.7%, preferablybetween 0.2 and 0.6%, and still more preferably between 0.2 and 0.5%. Asindicated below, the limits of these aluminum content ranges are greaterthan those of the bath used for making the coating 7. This is explainedby the formation of intermetallic substances at the junction between thesubstrate 3 and the coating 7 which leads to an increase in the aluminumcontent in the coating 7.

For making the metal sheet 1, it is possible for example to proceed asfollows.

A substrate 3 as a strip obtained for example by hot and thencold-rolling is used.

Preferably, for cold-rolling, one starts by cold-rolling the substrate 3with a reduction rate generally comprised between 60 and 85%, so as toobtain a substrate 3 with a thickness for example comprised between 0.2and 2 mm.

In a preferred embodiment, one makes sure that at least the lastcold-rolling pass is carried out with so-called “smooth” work rolls,i.e. rectified and non-etched rolls, for which the work surfaces have aroughness Ra_(2.5), i.e. measured with a cut-off threshold at 2.5 mm,less than 0.5 μm.

It is recalled that work rolls are the rolls of the rolling milldirectly in contact with the substrate 3 for ensuring its deformation.One refers, with the term of work surface, to their surfaces in contactwith the substrate 3.

The smooth work rolls will be present at least in the last cage(s) ofthe rolling mill when the running direction of the substrate 3 in therolling mill is considered.

The use of smooth work rolls at least for the last pass gives thepossibility of better controlling the waviness Wa_(0.8) of the metalsheet 1 obtained subsequently by coating of the substrate 3 on the onehand and parts which may be produced by deforming the metal sheet 1 onthe other hand.

In particular, such cold-rolling allows reduction in the wavinessWa_(0.8) as compared with rolling only resorting to rolls with strongerroughness, etched either by shot-blasting, or by an electric discharge(so-called Electron Discharge Texture (EDT) rolls), or further by anelectron beam (so-called Electron Beam Texture (EBT) rolls).

The cold-rolled substrate 3 may then be subject to annealing conductedin a conventional way in an annealing oven under a reducing atmosphere,with view to recrystallization after the work hardening which it hasundergone during the cold-rolling operation.

Recrystallization annealing further gives the possibility of activatingthe faces 5 of the substrate 3 so as to promote the chemical reactionsrequired for the subsequent dip-coating operation.

Depending on the grade of the steel, the recrystallization annealing iscarried out at a temperature comprised between 650 and 900° C., forexample, for a period required for recrystallization of the steel andfor activation of the faces 5.

The substrate 3 is then cooled to a temperature close to that of a bath13 contained in a tank 15.

The composition of the bath 13 is based on zinc and contains between,for example, 0.1 and 0.5% by weight of aluminum, preferably between 0.1and 0.4%, and still preferably between 0.1 and 0.3%.

The composition of the bath 13 may also contain up to 0.3% by weight ofoptional addition elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce,Cr, Ni, Zr or Bi.

These different elements may inter alia allow improvement in theresistance to corrosion of the coating or else its brittleness or itsadhesion, for example.

One skilled in the art who is aware of their effects on thecharacteristics of the coating will know how to use them according tothe sought additional purpose. It was also checked that these elementsdid not interfere with controlling the waviness obtained by the methodaccording to the invention.

Finally, the bath 13 may contain inevitable impurities from ingots forfeeding the tank or else further from the passage of the substrate 3 inthe bath 13. Mention may thus be notably made of iron.

After passing in the bath 13, the substrate 3 is coated on its two faces5 with the coating 7 for obtaining the metal sheet 1.

As illustrated by FIG. 2, the metal sheet 1 is then subject to wiping bymeans of nozzles 17 placed on either side of the metal sheet 1 and whichproject a wiping gas, for example air or an inert gas, towards the outersurfaces 21 of the coating 7. The wiping gas is ejected from each nozzle17 along a main ejection direction E. The main ejection directions E ofeach nozzle 17 are materialized in dotted lines in FIGS. 2 and 3.

In the illustrated example, the directions E are horizontal andorthogonal to the metal sheet 1. In other embodiments, the directions Emay have other inclinations relative to the metal sheet 1.

The running speed V of the substrate 3 on the production line used andtherefore in front of the nozzle 17 is generally comprised between, forexample, 80 m/min and 300 m/min, and it is preferably greater than 120m/min, or even 150 m/min.

In order to limit oxidation of the coating 7, provision may be made fora confinement box 23 for confining the atmosphere around the metal sheet1 downstream from the nozzle 17. The term of downstream is meant hererelatively to the running direction S of the metal sheet 1 facing thenozzles 17.

The confinement box 23 may alternatively be extended upstream as far asthe surface of the bath 13 or as far as an intermediate position betweenthe nozzle 17 and the surface of the bath 13.

In certain alternatives, the installation may not comprise anyconfinement box.

In the example described above, the nozzle 17 has structures, positionsrelative to the metal sheet 1 which are similar and they operate withsimilar adjustments. Thus, only the right nozzle 17 of FIG. 2 will bedescribed below, with reference to FIG. 3.

Alternatively, the nozzle 17 may have different structures, differentpositions and/or operate with different adjustments. It is also possibleto only provide a nozzle on one side of the metal sheet 1.

The nozzle 17 has an outlet 25 through which the wiping gas is ejectedtowards the outer surface 21 of the coating 7 placed opposite. Variousouter shapes may be contemplated for the nozzle 17.

The outlet 25 of the nozzle 17 is positioned at a distance Z from themetal sheet 1 along the main ejection direction E. As illustrated byFIG. 4, the outlet 25 generally appears as a slot which extends,perpendicularly to the running direction S and to the plane of FIG. 3,over a width L at least equal to the width of the metal sheet 1.

Generally, the height of the outlet 25, i.e. its dimension parallel tothe running direction S of the metal sheet 1 in front of the nozzle 17,is constant as illustrated by FIG. 4. This being the case, in certainalternatives, this height may vary over the width of the outlet 25.Thus, the outlet 25 may have for example a slightly flared shape towardsits end (shape of a bowtie).

In order to take into account these possible height variations and thedifferent possible embodiments, the average height d of the outlet 25 onits width L will be considered subsequently.

The pressure of the wiping gas in the nozzle 17 is noted as P and thevolume fraction of oxygen in the wiping gas is noted as fO₂.

According to the invention, at least one of the following equations isobserved:

$\begin{matrix}{{\frac{Z}{d} + {18\; {\ln \left( \frac{Z}{d} \right)}}} < {{8\; {\ln \left( \frac{P}{V} \right)}} - 27.52}} & (A) \\{{fO}_{2} < \frac{2.304 \cdot 10^{- 3}}{\left( {27.52 + \frac{Z}{d} + {8\; {\ln \left( {\frac{V}{P}\left( \frac{Z}{d} \right)^{2.25}} \right)}}} \right)^{2}}} & (B)\end{matrix}$

wherein:

Z is expressed in mm

d is expressed in mm

V is expressed in m·s⁻¹

P is expressed in N·m⁻²

In other words, if equation (A) is not observed, equation (B) has to beobserved and vice versa. The equations (A) and (B) may also be observedsimultaneously.

Generally, the parameters V and d are imposed by the production lineused. Therefore, there only remains Z and P or even fO₂, to be adjustedfor meeting the requirements above.

Parameters thus set give the possibility of attaining, aftersolidification of the coating 7 and before a possible skin-pass, awaviness Wa_(0.8) of less than or equal to 0.55 μm as illustrated byExample 1 below.

Still more advantageously, at least one of the following equations isobserved:

$\begin{matrix}{{\frac{Z}{d} + {18\; {\ln \left( \frac{Z}{d} \right)}}} < {{8\; {\ln \left( \frac{P}{V} \right)}} - 36.32}} & (C) \\{{fO}_{2} < \frac{2.304 \cdot 10^{- 3}}{\left( {36.32 + \frac{Z}{d} + {8\; {\ln \left( {\frac{V}{P}\left( \frac{Z}{d} \right)^{2.25}} \right)}}} \right)^{2}}} & (D)\end{matrix}$

wherein:

Z is expressed in mm

d is expressed in mm

V is expressed in m·s⁻¹

P is expressed in N·m⁻²

In other words, if equation (C) is not observed, equation (D) must beobserved and vice versa. The equations (C) and (D) may also be observedsimultaneously.

If the parameters Z, d, V, P and fO₂ satisfy equation (C) and/orequation (D) then, after solidification of the coatings 7 and before apossible skin-pass, a waviness Wa_(0.8) of less than or equal to 0.35 μmis attained.

The coatings 7 are then left to cool in a controlled way so that theysolidify.

As indicated earlier, at the end of this cooling operation 7 the outersurfaces 21 of the coating 7 have waviness values Wa_(0.8) of less than0.55 μm, or even less than 0.35 μm.

Alternatively, brushing may be carried out in order to remove thecoating 7 deposited on one face 5 so that only one of the faces 5 of thesubstrate 3 will finally be coated with a coating 7.

When the coatings 7 are completely cooled, the metal sheet 1 may undergoa skin-pass operation for giving texture to the outer surfaces 21 of thecoating 7, facilitating subsequent forming process of the metal sheet 1.

Indeed, the skin-pass operation gives the possibility of transferring tothe outer surfaces 21 of the coating 7 of the metal sheet 1 sufficientroughness in order for its forming process to be properly carried out,while promoting good retention of the oil applied on the metal sheet 1before it is formed. The elongation rate of the metal sheet 1 during theskin-pass operation is generally comprised between 0.5 and 2%.

Preferably, the skin-pass operation will give the possibility of keepinga waviness Wa_(0.8) of less than 0.55 μm and preferably less than 0.35μm for the outer surfaces 21 of the coating 7.

In a first alternative, the skin-pass operation will be carried out withEDT work rolls for which the work surfaces have a roughness Ra_(2.5)comprised between 2.05 and 2.95 μm. If the elongation rate during theskin-pass operation is less than or equal to 1.1%, the roughnessRa_(2.5) of the work surfaces of the EDT work rolls will preferably becomprised between 2.50 and 2.95 μm. If the elongation rate during theskin-pass operation is greater than or equal to 1.1%, the roughnessRa_(2.5) of the work surfaces of the EDT work rolls will preferably becomprised between 2.05 and 2.50 μm.

In another alternative, the skin-pass operation will be carried out withEBT work rolls for which the work surfaces have a roughness Ra_(2.5)comprised between 2.90 and 4.10 μm. If the elongation rate during theskin-pass operation is less than or equal to 1.1%, the roughnessRa_(2.5) of the work surfaces of the EBT work rolls will preferably becomprised between 3.50 and 4.10 μm. If the elongation rate during theskin-pass operation is greater than or equal to 1.1%, the roughnessRa_(2.5) of the work surfaces of the EBT work rolls will preferably becomprised between 2.90 and 3.50 μm.

The skin-pass operation is generally carried out for a metal sheet 1intended for manufacturing body parts for automobiles.

When the metal sheet 1 is intended for manufacturing domestic electricappliances, for example this additional operation is not carried out.

The metal sheet 1 having been skin-passed or not may then be cut out andthen undergoes a forming process, for example by drawing, bending orprofiling, in order to form a part which may then be painted in order toform, on each coating 7 a paint film (or a paint system).

In the case of parts for domestic electrical appliances, it is alsopossible to possibly subject the paint films to annealing with physicaland/or chemical means, known per se.

For this purpose, it is possible to have the painted part pass through ahot air or induction oven, or further under UV lamps or under a devicediffusing electron beams.

After deformation, the outer surfaces of the coating 7 of the part havea waviness Wa_(0.8) of less than or equal to 0.60 μm, or even less thanor equal to 0.45 μm, or even less than or equal to 0.43 μm, or even 0.41μm or even further 0.37 μm.

This waviness may for example be measured after 3.5% equi-biaxialdeformation.

Controlling the waviness Wa_(0.8) before a possible skin-pass and aftera possible skin-pass to values of less than or equal to 0.55 μm,respectively 0.35 μm, as described above, allows control of the wavinessWa_(0.8) after deformation to values of less than or equal to 0.60 μm,respectively 0.45 μm, 0.43 μm, 0.41 μm or even 0.37 μm.

For automotive applications, after phosphate-coating, each part isdipped in a cataphoresis bath, and a primer paint layer, a base paintlayer, and optionally a finishing varnish layer are applied insuccession.

Before applying the cataphoresis layer on the part, the latter isdegreased beforehand and then phosphate-coated so as to ensure theadherence of the cataphoresis.

The cataphoresis layer provides the part with additional protectionagainst corrosion. The primer paint layer, generally applied with a gun,prepares the final appearance of the part and protects it against stonechipping and against UVs. The base paint layer gives the part its colorand its final appearance. The varnish layer imparts to the surface ofthe part, good mechanical strength, resistance against aggressivechemical agents and a good surface aspect.

Generally, the weight of the phosphate coating layer is comprisedbetween 1.5 and 5 g/m².

The paint films applied for protecting and guaranteeing an optimumsurface aspect to the parts, for example comprise a cataphoresis layerwith a thickness from 15 to 25 μm, a coat of primer paint with athickness from 35 to 45 μm, and a base coat of paint with a thicknessfrom 40 to 50 μm.

In the cases when the paint films further comprise a varnish layer, thethicknesses of the different paint layers are generally the following:

cataphoresis layer: between 15 and 25 μm, preferably less than 20 μm,

primer paint layer: less than 45 μm,

base paint layer: less than 20 μm, and

varnish layer: less than 55 μm.

The paint films may also not comprise any cataphoresis layer, and onlycomprise a primer paint layer and a base paint layer and optionally avarnish layer.

Preferably, the total thickness of the paint films will be less than 120μm or even 100 μm.

The invention will now be illustrated by tests given as an indicationand not as a limitation.

Example 1 Influence of the Volume Fraction fO₂

The tests conducted in this example aim at showing the positiveinfluence of observing equations (A) and/or (B), or even (C) and/or (D).

The table I below details the conditions of a series of tests conductedwith different values of Z, d, V, P and fO₂ and provides the wavinessesWa_(0.8) measured before a skin-pass, NSKP meaning not having beenskin-passed.

The procedure for measuring the waviness Wa_(0.8) consists of acquiringby mechanical probing (without any shoe) a metal sheet profile with alength of 50 mm, at 45° from the rolling direction. From the signalobtained by probing, the approximation of its general shape with apolynomial of a degree of at least 5 is subtracted. The waviness Wa andthe arithmetic mean roughness Ra is then separated by a Gaussien filterby applying a cut-off of 0.8 mm.

The right columns specify for each test, whether the parameters satisfythe equations (A), (B), (C) and (D).

TABLE I Wa_(0.8) Z d V P NSKP Eq Eq Tests (mm) (mm) (m/min) (N/m2) fO₂(μm) Eq (A) Eq (B) (C) (D) 1 9 1.2 150 38800 0.21 0.36 yes yes no no 211 1.2 150 48200 0.21 0.47 yes yes no no 3 7 1.2 150 26300 1.10⁻⁴ 0.27yes yes yes yes 4 9 1.2 150 38800 1.10⁻⁴ 0.31 yes yes no yes 5 13 1.2150 59000 1.10⁻⁴ 0.44 no yes no no 6 8 1 120 33100 0.21 0.43 yes yes nono 7 10 1 120 40700 0.21 0.50 yes yes no no 8 14 1 120 60900 0.21 0.84no no no no 9 6.5 1 120 32600 1.10⁻⁴ 0.31 yes yes yes yes 10 10 1 12052000 1.10⁻⁴ 0.41 yes yes no no 11 14 1 120 64900 1.10⁻⁴ 0.66 no no nono 12 8 1.5 100 22400 1.10⁻⁴ 0.31 yes yes yes yes 13 15 1.5 100 408001.10⁻⁴ 0.37 yes yes no no

Thus, the use of parameters satisfying equation (A) and/or (B) gives thepossibility of attaining waviness values before a skin-pass Wa_(0.8) ofless than 0.55 μm.

The use of parameters satisfying equation (C) and/or (D) gives thepossibility of attaining wavinesses before a skin-pass Wa_(0.8) stillsmaller and less than or equal to 0.35 μm.

Waviness values before any skin-pass Wa_(0.8) of less than or equal to0.35 μm may in certain cases be reached without observing equations (C)and/or (D), notably by observing equations (A) and/or (B) and by usingsmooth work rolls for the cold-rolling and/or of a particular roughnessfor the skin-pass, as discussed below.

Example 2 Influence of Cold-Rolling with Smooth Work Rolls

The tests conducted in this example aim at showing the positiveinfluence of cold-rolling carried out with smooth work rolls, ascompared with rolling carried out with EDT work rolls, the work surfaceof which have a greater roughness.

For this purpose, steel substrates are subject to cold-rolling in orderto attain a thickness of 0.8 mm, either by using so-called smooth workrolls, the work surfaces of which have a roughness Ra_(2.5) of 0.5 μm,or EDT work rolls for which the work surfaces have a roughness Ra_(2.5)of 3 μm. The substrates 3 are then coated with a zinc coating by hot dipcoating in a zinc bath comprising 0.18% by weight of aluminum, for whichthe temperature is 460° C., and are wiped with nitrogen so as to form azinc coating having a thickness of 6.5 μm.

After complete cooling of the thereby obtained metal sheet 1, the latterare subject to a skin-pass operation conducted with EBT etched workrolls, the work surfaces of which have a roughness Ra_(2.5) of 5 μm,before being cut out and formed by drawing.

The waviness values Wa_(0.8) of the outer surfaces 21 of the coating 7are measured at the end of each of the steps of the method, i.e. aftercold-rolling (CR), after the skin-pass operation (SKP) and after forming(DEF). The latter is carried out by equi-biaxial deformation of 3.5%with a Marciniak tool.

The results of the measurements of Wa_(0.8) are grouped in table II.

As may be seen, the use of smooth rolling allows reduction in thewaviness Wa_(0.8) regardless whether this is at the end of thecold-rolling, of the skin-pass or of the forming step.

TABLE II Wa_(0.8) Wa_(0.8) Wa_(0.8) Ra_(2.5) (μm) (μm) (μm) (μm) workrolls after after after Eq Tests CR CR SKP DEF (A) Eq (B) Eq (C) Eq (D)14 3 0.52 0.39 0.41 yes yes no no 15 0.5 0.15 0.35 0.34 yes yes no no

Example 3 Influence of the Skin-Pass

The tests carried out in this example aim at ensuring the positiveinfluence of a skin-pass carried out by using work rolls, for which thework surfaces have a certain roughness Ra_(2.5).

For this purpose, steel substrates 3 were subject to cold-rolling inorder to form cold-rolled substrates for which the thickness is 0.7 mm.

The substrates 3 are then coated with a zinc coating by hot dip coatingin a zinc bath comprising 0.18% by weight of aluminum, the temperatureof which is 460° C., and are dried with nitrogen in order to form zinccoatings having a thickness of 6.5 μm.

The thereby obtained metal sheets 1 are divided into two batches.

Before being formed by equi-biaxial deformation of 3.5% with a Marciniaktool, the metal sheets 1 from the first batch are subject to a skin-passcarried out with EDT work rolls and with an elongation rate of 1.4%. Theroughness Ra_(2.5) of the work surfaces is 2.20 μm.

The metal sheets 1 from the second batch are subject to a skin-passoperation with the same elongation rate but with EDT etched work rolls,for which the work surfaces have a roughness Ra_(2.5) of 2.60 μm.

The results of the tests are grouped in table III.

TABLE III Wa_(0.8) Wa_(0.8) Wa_(0.8) Ra_(2.5) (μm) (μm) (μm) (μm) workrolls after after after Eq Tests SKP CR SKP DEF (A) Eq (B) Eq (C) Eq (D)16 2.20 0.42 0.28 0.37 yes yes no no 17 2.60 0.42 0.41 0.47 yes yes nono

What is claimed is: 1-18. (canceled)
 19. A method for manufacturing ametal sheet comprising a steel substrate, at least one face of which iscoated with a metal coating comprising Al, a remainder of the metalcoating being Zn and inevitable impurities, the metal coating having aweight content of aluminum from 0.2 to 0.7%, the method comprising thesteps of: providing the steel substrate; depositing a metal coating onat least one face of the steel substrate by dipping the steel substratein a bath to obtain the metal sheet; wiping the metal coating with awiping gas from at least one nozzle projecting through at least oneoutlet, the metal sheet running in front of the at least one nozzle, thewiping gas being ejected from the nozzle along a main ejection directionE; solidifying the metal coating, an outer surface of the metal coatinghaving, after solidification and before an optional skin-pass operation,a waviness Wa_(0.8) of less than or equal to 0.55 μm; and satisfying atleast one of the following equations: $\begin{matrix}{{\frac{Z}{d} + {18\; {\ln \left( \frac{Z}{d} \right)}}} < {{8\; {\ln \left( \frac{P}{V} \right)}} - 27.52}} & (A) \\{{fO}_{2} < \frac{2.304 \cdot 10^{- 3}}{\left( {27.52 + \frac{Z}{d} + {8\; {\ln \left( {\frac{V}{P}\left( \frac{Z}{d} \right)^{2.25}} \right)}}} \right)^{2}}} & (B)\end{matrix}$ wherein: Z is a distance between the metal sheet and thenozzle along the main ejection direction E, Z being expressed in mm, dis an average height of the outlet of the at least one nozzle along arunning direction S of the metal sheet in front of the nozzle, d beingexpressed in mm, V is a running speed of the metal sheet in front of theat least one nozzle, V being expressed in m·s⁻¹, P is a pressure of thewiping gas in the at least one nozzle, P being expressed in N·m⁻², andfO₂ is a volume fraction of oxygen in the wiping gas.
 20. The methodaccording to claim 19, wherein the metal coating further includes one ormore additional elements selected from the following: Si, Sb, Pb, Ti,Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, a weight content of each additionalelement in the metal coating being less than 0.3%.
 21. The methodaccording to claim 19, wherein the waviness Wa_(0.8) of the outersurface of the metal coating after solidification and before an optionalskin-pass operation is less than or equal to 0.35 μm.
 22. The methodaccording to claim 19, further comprising the step of: skin-pass rollingthe metal sheet with EDT work rolls, the working surface having aroughness Ra_(2.5) from 2.05 to 2.95 μm.
 23. The method according toclaim 1, further comprising the step of: skin-pass rolling the metalsheet with EBT work rolls, the working surface having a roughnessRa_(2.5) from 2.90 to 4.10 μm.
 24. The method according to claim 19,wherein the metal coating has a weight content of aluminum less than orequal to 0.6%.
 25. The method according to claim 24, wherein the metalcoating has a weight content of aluminum less than or equal to 0.5%. 26.A metal sheet comprising: a steel substrate having at least one face;and a metal coating the at least one face of the substrate, the metalcoating being deposited by dipping the steel substrate in a dip bath;the metal coating comprising between 0.2 and 0.7% of Al, a remainder ofthe metal coating being Zn and inevitable impurities, an outer surfaceof the metal coating having a waviness Wa_(0.8), before an optionalskin-pass operation, of less than or equal to 0.35 μm.
 27. The metalsheet according to claim 26, wherein the metal coating further includesone or more additional elements selected from the following: Si, Sb, Pb,Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, a weight content of eachadditional element in the metal coating being less than 0.3%
 28. Themetal sheet according to claim 26, wherein the metal coating has aweight content of aluminum less than or equal to 0.6%.
 29. The metalsheet according to claim 28, wherein the metal coating has a weightcontent of aluminum less than or equal to 0.5%.
 30. A part comprising: ametal sheet according to claim 26 formed into a part, the outer surfaceof the metal coating having a waviness Wa_(0.8) of less than or equal to0.43 μm.
 31. The part according to claim 30, wherein the outer surfaceof the metal coating has a waviness Wa_(0.8) of less than or equal to0.41 μm.
 32. The part according to claim 31, wherein the outer surfaceof the metal coating has a waviness Wa_(0.8) of less than or equal to0.37 μm.
 33. The part according to claim 30, further comprising a filmof paint on the metal coating.
 34. The part according to claim 33,wherein a thickness of the film of paint is less than or equal to 120μm.
 35. The part according to claim 34, wherein the thickness of thefilm of paint is less than or equal to 100 μm.
 36. A land motor vehiclecomprising: a body, the body including a part according to claim
 30. 37.The method according to claim 19, wherein at least one of the followingequations is satisfied: $\begin{matrix}{{\frac{Z}{d} + {18\; {\ln \left( \frac{Z}{d} \right)}}} < {{8\; {\ln \left( \frac{P}{V} \right)}} - 36.32}} & (C) \\{{fO}_{2} < \frac{2.304 \cdot 10^{- 3}}{\left( {36.32 + \frac{Z}{d} + {8\; {\ln \left( {\frac{V}{P}\left( \frac{Z}{d} \right)^{2.25}} \right)}}} \right)^{2}}} & (D)\end{matrix}$
 38. The method according to claim 37, further comprising,before the deposition step, a step of: cold rolling the metal sheet, atleast a last pass of the cold rolling being achieved with rectified andnon-etched work rolls for which work surfaces have a roughness Ra_(2.5)of less than or equal to 0.5 μm.
 39. The method according to claim 19,further comprising, before the deposition step, a step of: cold rollingthe metal sheet, at least a last pass of the cold rolling being achievedwith rectified and non-etched work rolls for which work surfaces have aroughness Ra_(2.5) of less than or equal to 0.5 μm.